Wave transmission network



Sept 5, 195o B. M. OLIVER 2,521,125

WAVE TRANSMISSION NETWORK Filed Aug. 21, 1947 2 Sheets-Sheet 1 IAS/@2 c4 nR/ELm/s INPUT I I REVERS/NG sw/TcH oPERA T50 Ar MRR/rn meal/Enr /NVEA/ron B. M. OLIVER sePt- 5, 1950 B. M. OLIVER 2,521,125

WAVE TRANSMISSION NETWORK Filed Aug. 21, 1947 2 sheets-sheet 2 F/G. 4 F/G. 4A

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C R L A '/NVE/Vron By BM. OL/l/ ER ATTORNFV Patented Sept. 5, 1950 WAVE TRANSMISSION NETWORK Bernard M. Oliver, vNew York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application August 21, 1947, Serial No. '769,853

(Cl. Tis- 44) 14 Claims.

This invention relates to transmission networks, particularly to two terminal impedence equalizers.

An object of the invention is to shift the impedance characteristic of a network along the frequency scale without altering the shape of said impedance characteristic.

A further object of the invention is to maintain the impedance characteristic of a two-terminal network symmetrical about a carrier despite fluctuations in the frequency of this carrier.

A feature of the invention is a network which provides impedance equalization about a carrier frequency by switching of the .polarity of ouru rents impressed thereon at the carrier frequency.

Another feature of the invention is the provision of two-terminal networks whose impedance characteristics vary sharply in the neighborhood of a carrier frequency.

Referring to the figures of the drawing:

Fig. 1 shows an impedance network including a polarity switching diode bridge;

Fig. 2 represents the corresponding impedance characteristic;

Fig. 3 illustrates schematically the circuits of the invention in their broader aspects.

Fig. 3A shows an amplifier circuit in which the circuit of Fig. l is incorporated.

Figs. fl, 5 and 6 show modified component impedances for use with the diode bridge.;

Figs. 4A, 5A, 6A, represent the impedance characteristics obtained with the diode bridge and the impedances of Figs. 4, 5, 6, respectively; and

Fig. 7 is a further modification of the improved network inaccordance with the invention.

In equalizing servo systems as well as in other applications, it is often necessary to obtain a transmission characteristic which varies sharply in the neighborhood of the carrier frequency being used. Attempts to obtain such a characu teristic by means of networks formed of resonant and anti-resonant circuits have heretofore enn countered difficulties when the required sharpness of the desired transmission characteristic is increased, to wit:

(1) The operation of the equalizer network be-` comes increasingly dependent on the frequency stability of the carrier.

(2) The required Qs of the resonant circuit components become higher than ordinarily obtainable in practice.

in case such difficulties prove too great, an alternative method for securing the desiredtransmission characteristics about the carrier is todemodulate the signal from the carrier and equalize at signal frequencies, and then to remodulate up to the carrier again. This demodulation and remodulation can be accomplished either electrically or mechanically depending on the carrier frequency used.

In a broad aspect, the invention contemplates the connection of an impedance element or network to a circuit carrying waves modulated upon a carrier wave, through a, reversing or commutating switch, which operates at the frequency of the carrier wave, each time the carrier wave passes through zero or an approximately zero value.

vin accordance with an embodiment of the present invention, two-terminal networks with impedance characteristics about a median frequency are achieved, characterized by a capability of shifting in the frequency scale without change of shape as the carrier fluctuate.; or

shifts. This is accomplished by commutating the polarity of the input to a two-terminal impedance of conventional form by means of a bridge of unilaterally conductive elements operated at a carrier frequency from a source connected to one 4diagonal thereof. A signal applied to the other diagonal will encounter an impedance characteristic developed by the combination of twoterminal reactance and bridge which will shift lalong with shifts in the carrier frequency. From another aspect, the network in accordance with the invention provides impedance equalization by electrical demodulation of the carrier, equalization at signal frequencies and remodulation on a reflux basis, so that the same elements serve both as the demodulating and remodulating means, to thereby effect a reduction of, and economy in, parts.

Referring to Fig. 1,'the equalizer network in accordance with the inventio-n comprises in combination an impedance network l and a diode bridge 2, commutated at carrier frequencies.

The bridge 2 is of the type normally used as a demodulator or phase detector, such as is disclosed in the United yStates Patent No. 2,086,601, issued July 13, 1937, to R. S. Carruthers. The bridge 2 comprises identical diode tubes 3 in the arms thereof as shown in Fig. 1, each diode being in series with a resistor Ro. In lieu of diodes, other unilateral conductive devices may be used such as cuprous oxide rectiers, selenium rectiers, silicon or germanium crystals and the like.

In the diode bridge 2, two inputs are provided, namely, a signal input Il and la carrier input 5, applied to the diagonale of the bridge i-B and 9-l0, respectively, by the transformers Il, I2

3 as shown in Fig. l. A two-terminal impedance network I which, by way of example, is shown as comprising a condenser C and resistor R. in parallel, is inserted between center taps I3 and I4 of the secondary coils. Other forms of two-terminal impedances may be used as subsequently disclosed in Figs. 3, 4, 5. These impedance networks or impedance branches are made up of linear-bilateral reactive and resistive components. Linear-bilateral is thus taken to describe a component which presents the same impedance characteristic to current pulses flowing in either direction through the component. An impedance branch comprising these components may be termed symmetrically conductive, meaning that the current response of the branch is independent of the polarity of the applied voltage and that the average current through said branch over a complete cycle of applied alternating voltage is zero.

The combination of circuits comprising component impedance I, bridge 2, and carrier circuit 5, represents a two-terminal network, whose impedance characteristic as seen by the signal source I will automatically follow fluctuations or frequency changes in the carrier The mode of operation of the circuit in accordance with the invention and the nature of the transmission characteristic provided thereby for equalization or other purposes may be more fully understood from the following analysis:

Assume in the circuit of Fig. 1 that each resistor R0 is made much greater than the resistance of its associated diode 3 in the conducting direction. For the purpose of analysis, the diode resistance is included as part of Ro. Normally the resistance of the diodes 3 in the non-conducting direction is substantially infinite, as is well known. The carrier amplitude is assumed to lbe larger than the signal amplitude, the latter being derived from a high impedance source I5.

Under these conditions, the carrier of frequency performs a switching operation in the bridge 2, namely, it renders either the top two diodes or the bottom two diodes of Fig. 1 conducting upon alternate half cycles.

If at :0, sinusoidal current of frequency Liq 27r is suddenly applied to the signal input at 4, then the current flowing iny either` half of the secondary may be represented as i=I1 (sin wot) (1) If this signal at 4 is in phase with the carrier input at 5, and under the assumption that the reactance of C at wn is much less than R, i. e., woCR 1, then the charge stored in condenser C each half cycle will be and the average current this represents is:

9g Aqu0 2 I-l--7T TII (3) The Voltage across C due to this current is obviously: E=IR(1' 5")=%1R(1 6-51) wherein 1=10 This voltage gets switched from one-half of the balanced secondary of the signal input transformer I I to the other every half cycle. The amplitude, i. e., the half peak to peak of the square wave thus produced across each half of the secondary is also E, and the fundamental component of the square wave has the amplitude The total fundamental voltage across each vhalf \\m This is the same wave as would have been obtained had the signal current I1 sin wot been applied to an impedance Z(p where 8 2 R0 P *i* 51+ ;j2 t

In other words the signal equalization which will be achieved by the two-terminal network at terminals 4 (Fig. 1) is expressible, on a modulation frequency basis, as the impedance characteristic graphically shown in Fig. 2, Here =ww0, the modulation frequency and On a carrier basis, the impedance presented by the two-terminal network at terminals 4 is the characteristic curve or graph shown in Fig. 2 mirrored about the carrier frequency. Should the carrier frequency shift, the equalizer characteristic would also shift so that the effective impedance at the carrier frequency would always be R0 8 -I-ER and, remote from the carrier frequency, simply the corners being at Luo-62, wo-i, wo-l-i, wo-l-z. With a square wave signal input, the same sort of analysis can be made with substantially the same results except that the disappears from the previous equations and from the expression of the effective impedance above. The impedance looking into the primary of the input coil II is of course times that seen by each half of the secondary, wherein n is the turns ratio of the transformer. Fig. 3 illustrates schematically the broad aspects and operating principles in accordance with the invention.

The rectangle A represents a multifrequency source, such as a source of modulated waves. In one specific application, the source may, for example, be the cathode-anode terminals of a pentode tube in a multistage amplifier, and B may be the following interstage circuit.

The reversing switch C, which operates at the carrier frequency and reverses whenever the carrier passes substantially through zero value, corresponds to the diode bridge 2 of Fig. 1, switched by the carrier input 5. The network D corresponds to the impedance network I, illustrated in Fig. l, or tc the various forms of im- Y pedance networks illustrated in Figs. 4, 5, 6 rcspectively.

Fig. 3A shows schematically the circuit of Fig. l incorporated in a form of multistage aliiplirier described with respect to Fig. 3. The anode current from the pentode tube P supplies the signal frequencies to the primary of transformer Il, and thence to the circuit of Fig. 1, as represented by Q, the dotted line box. The pentode P is coupled to the interstage circuit in the conventional manner.

Various impedance characteristics, such as, for example, those illustrated in Figs. 4A, 5A and 6A, may be obtained by the use of substitute networks shown in Figs. 4, 5, 6, respectively, in the direct current branch l3--4 of the equalizer. Examples of impedance components which may be used in lieu of the RC circuit of Fig. 1 are presented in Figs. 4, 5, 6, to Wit, a single condenser C, a two-terminal network of parallel impedances R2, C, R1 C2 or an L, C, R -parallel tuned circuit. These networks should all have a driving point impedance W compared with Rn at the carrier frequency where Zum-wo) is the impedance of the network used at the frequency =ww0- The figures show Zw) plotted against tti-wn. It is also possible to remove the input transformer l I and to replace the RC network in the branch I2-I4 (Fig. 1) with two networks of twice the impedance, as shown in Fig. 6, connected from one of the signal input leads 20, 2l to each of the corners 1-8 of the bridge 2.

The circuit of Fig. 6 may be further modiecl by replacing the two resistances 2R across the condensers C/2 by a single resistance shunting the input terminals 20, 2l. For a square wave, the value of the single resistor would be RR@ R-R For a sine wave input, the required value for the single resistor would be 8 RRU Various forms of switching device for commutating the polarity may be utilized in accordance with the invention, such as a relay, vibrat- 6 ing reed, gas tube switch, vacuum tube sii'ritcli or the like.

It should also be understood that applicants improved two-terminal impedance network pre'- viously described may be incorporated as a cornponent element in the synthesis of generalized four-terminal networks, without departing from the spirit of the invention.

What is claimed is:

l. A two-terminal impedance device comprising a symmetrically-conductive branch of at least one linear-bilateral impedance, means for applying a Signal thereto, and means for periodically commutating the polarity of said signal, whereby the impedance characteristic may shift as the period of commutation fluctuates.

2. A two-terminal impedance device comprising in combination a bridge of unilaterally conductive devices, a source of carrier frequency for commutating the conductivity of said devices in pairs, said source being connected to one diagonal, and a symmetrically-conductive impedance branch of the type described connected to the bridge, and having a signal source operatively connected to said two-terminal impedance.

3. A two-terminal impedance device comprising a bridge of unilaterally conductive devices connected thereto, an impedance network connected to the bridge, said network being twoterminal and having a current response thereof independent of polarity of an applied voltage, an alternating current source connected to said bridge for commutating the conductivity of said bridge periodically and controlling the polarity of a second current applied to said impedance network.

4. The structure of claim. 3, wherein said currents have phase relation of an integral number of 1r radians and have a common frequency.

5. In combination a two-terminal impedance device comprising a bridge of unilateral conductive elements, an impedance branch consisting of linear-bilateral components connected thereto, and means for periodically commutating the conductivity of pairs of said elements, and a source of signals, whose frequency range includes said period of commutation, applied to said bridge and impedance branch.

6. A two-terminal impedance device adapted to be operated about a mean carrier frequency comprising a symmetrically-conductive impedance branch and a bridge, means for rendering each half of said bridge alternately conductive and non-conductive at the carrier frequency, whereby asymmetrical impedance characteristic about said carrier is developed.

7. The structure of claim 6 and a separate source of signals applied to said bridge, said signals including the carrier frequency.

8. A two-terminal impedance device comprising an impedance branch comprising linear-bilateral components such that the average current through said branch is substantially zero for complete cycle of an alternating applied voltage, a source of multifrequency signals applied thereto, periodic switching means adapted to control the polarity of said signals applied to said impedance branch, said period being contained in said multifrequency source, whereby said impedance characteristic may shift unaltered in shape with fluctuations in said period.

9. In combination, a source of modulated carrier wave signals, a circuit connected thereto, and a symmetrically-conductive impedance branch connected to said circuit through a re- 7. versing switch, said switch being operated periodically at the frequency of said carrier wave.

10. The structure of claim 9, wherein said impedance branch is a single reactance.

11. The structure of claim 9, wherein said irnpedance branch is resonant.

12. In combination, a source of modulated carrier wave signals, a transmission circuit for said signals connected to said source, and a symmetrically-conductive branch of at least one linear-bilateral impedance connected to said circuit, a commutating switch associated with said branch and adapted to operate periodically at the frequency of said carrier wave.

13. In combination, a signal source of a first frequency, a symmetrically-conductive impedance branch, a. signal source of a second frequency, and means including said second source for commutating signal of said first source at a frequency of said second signal source and applying said commutated signal to said impedance branch, whereby said branch presents an impedance to said signal source dependent upon the difference between said first frequency and said second frequency.

14. In combination, a source of multifrequency signals comprising a modulation frequency modulated upon a carrier wave subject to substantial frequency Variations, a load connected to said source, means for presenting a predetermined impedance characteristic to said signals independent of said carrier variations, comprising an impedance branch connected to said load, a reversing switch interposed between said branch and said load, and a source of signals having the frequency of said carrier and subject to said same frequency variations thereof separately connected to said switch to operate said switch at said carrier frequency, whereby an impedance characteristic symmetrical about said carrier is presented to said signal as said carrier uctuates.

BERNARD M. OLIVER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,293,628 Reiling Aug. 18, 1942 FREIGN PATENTS Number Country Date 460,238 Great Britain Jan. 25, 1937 

